Magnetic mixer

ABSTRACT

A magnetic mixer for laboratory use which includes a permanent magnet mounted for rotation and driven on an axis centrally between its poles. A volume of combined liquids, containing in addition a multiplicity of magnetic particles, is supported centrally with reference to the magnet poles for activation of the particles in the rotating magnetic field to mix the liquids to achieve a reaction between them or merely a blending. This rotational force on the mixture has the effect of redistributing the magnetic particles away from the center of the mixture which effect, unless countered, lessens the mixing action. A second permanent magnet is provided having its poles in substantial alignment with the first mentioned axis, and having one of these last-mentioned poles spaced a distance nearer to the mixture than the other pole thereof, the magnetic field of the second magnet being substantially at right angles to the magnetic field of the first magnet. The field of the second magnet effectively counters by attraction the aforementioned redistributing movement of the magnetic particles, and the resultant particle distribution is substantially uniform throughout the combined liquids.

United States Patent [1 1 Lichtenstein Aug. 14, 1973 MAGNETIC MIXERInventor: Bernard Lichtenstein, Yorktown Heights, N.Y.

Assignee: Technicon Instruments Corporation,

Tarrytown, N.Y.

Filed: Dec. 13, 1971 Appl. No.: 207,196

[ 52] U.S. Cl. 259/1 R, 259/DIG. 46, 356/39,

[5 6] References Cited UNITED STATES PATENTS 3,2l9,3l8 11/1965 Hershler..259/1R Primary Examiner-John Petrakes Assistant Examiner-Alan l.Cantor Attorney-Saverio P. Tedesco [57] ABSTRACT A magnetic mixer forlaboratory use which includes a Int. Cl. com 21/24,];011' 13/08 Field ofSearch 259/1 R, 99, mo. 46; 356/39, 208

permanent magnet mounted for rotation and driven on an axis centrallybetween its poles. A volume of combined liquids, containing in additiona multiplicity of magnetic particles,v is supported centrally withreference to the magnet poles for activation of the particles in therotating magnetic field to mix the liquids to achieve a reaction betweenthem or merely a blending. This rotational force on the mixture has theeffect of redistributing the magnetic particles away from the center ofthe mixture which effect, unless countered,

lessens the mixing action. A second permanent magnet is provided havingits poles in substantial alignment with the first mentioned axis, andhaving one of these last-mentioned poles spaced at distance nearer tothe mixture than the other pole thereof, the magnetic field of thesecond magnet being substantially at right angles to the magnetic fieldof the first magnet. The field of the second magnet effectively countersby attraction the aforementioned redistributing movement of the magneticparticles, and the resultant particle distribution is substantiallyuniform throughout the combined liquids.

13 Claims, 6 Drawing Figures I Q l BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates to a magnetic mixer for mixingtogether plural combined volumes of liquid in which a volume of magneticparticles have been added for activation and mixing action in theliquid.

2. Prior Art Especially in clinical laboratories, activated magneticparticles have been used both for mixing together two combined liquids,such as a volume of sample and a volume of reagent, and as areaction-intensifying agent. These uses may be combined or concomitantas in the coagulation testing of human blood for Prothrombin Time (PT)or Partial Thromboplastin Time (PTT) as illustrated and described inAdler U. S. Pat. No. 3,650,698, issued Mar. 21, 1972, assigned to theassignee of the instant application.

In the last mentioned usage, a movable tape is employed. The tapecontains predeposited spots of an iron oxide material in the form ofopaque magnetized particles suspended in a water soluble media inthermoformed wells or on a planar tape surface. The testing of thesample blood plasma takes place in each thermoformed well on the clearplastic tape. The plasma and reagents are added at specific points asthe tape transports the mixture to the optical detector site. Themagnetic particles are resuspended in this solution which is incubatedat 37.5 C. for approximately one minute. The plasma and the magneticparticles become mixed by activation of the particles by a rotarypermanent magnet. The addition of the final reagent activates an opticaldetector and a timing device. Constant mixing is achieved by use of therotating magnetic field of the magnet. The suspension presented to theoptical detector should be uniformly opaque. This is a function of theiron particles. Fibrin formation from fibrinogen present in the plasmais recognized when the opaque suspension very abruptly becomestransparent as one or more forming fibrin strands entrap the magneticiron particles and sweep them together into small localized spots orclumps in the transparent liquid. In other words, the fibrin and all theparticles coalesce into a small mass, thus rendering the formerly opaquesuspension transparent. The optical detector or monitoring system stopsthe timer by a signal upon this optical change, and the elapsed time isprinted out.

The success of this system depends, among other things, upon theaccuracy of the optical detector in detecting the end reaction which isthe formation of fibrin. Lack of substantial uniformity of distributionof the magnetic particles in the sample during mixing prior to theformation of fibrin not only lessens the mixing action of the particlesbut concomitantly renders the sample of nonuniform opaqueness. Suchnonuniformity of opaqueness results, in the use of the optical detector,in false or premature cutoffs of the timer.

Such a phenomenon is unlikely to occur if the concentration of the ironoxide in the sample is maintained at a relatively high level or asdisclosed in the aforementioned US. Pat. No. 3,650,698. However, it hasbeen found that relatively high concentrations of iron oxide have anadverse effect on determinations of PT. In the use of such apparatuswith a lower concentration of iron oxide in the sample to avoid suchinterference with the PT determination, the rotating magnetic field hasredistributed the magnetic particles in solution, as by centrifugal andfluid forces, to render the mixture nonuniform and lacking uniformity ofopaqueness prior to the end reaction, so as to cause false or prematurecutoffs of the timer by signals generated by the optical detector.

SUMMARY OF THE INVENTION The invention has for an object the provisionof a magnetic mixer which effectively tends to evenly distributemagnetic particles through a liquid medium in which two or more liquidsare to be mixed, enhancing the mixing action of such particles. Afurther object is to provide such a mixer which tends to distribute arelatively low concentration of iron oxide particles in a blood plasmasample in a manner such that the sample is rendered substantiallyuniformly opaque during coagulation testing, prior to the end reactionto be detected by an optical detector.

There is provided a magnetic mixer which, while not limited thereto, isespecially useful in laboratories in carrying out various types ofanalysis on various typesofliquid substances, which mixer includes apermanent magnet mounted for rotation and driven on an axis centrallybetween its poles. A volume of sample-reagent liquid, containing inaddition a multiplicity of magnetic particles, is supported centrallywith reference to the magnet poles for activation of the particles inthe rotating magnetic field to mix the sample-reagent liquid to achievea reaction between these constituents or merely for blending themtogether. To oppose the centrifugal and fluid forces on the mixture bythe rotating field, which has the effect of redistributing the magneticparticles away from the center of the mixture, a second permanent magnetis provided having its poles in substantial alignment with the firstmentioned axis and having one of these poles spaced a distance nearer tothe mixture than the other pole thereof, the magnetic field of thesecond magnet being substantially at right angles to the magnetic fieldof the first magnet.

BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 is a fragmentary view in elevation and partially in sectionillustrating somewhat diagrammatically mixing apparatus embodying theinvention;

FIG. 2 is a fragmentary top plan view illustrating diagrammatically atape-supported volume of a liquid medium in which there is one type ofunequal or nonuniform magnetic particle distribution as influenced by asingle magnet (not shown), which generates a rotating magnetic field, inaccordance with the prior art;

FIG. 3 is a view similar to FIG. 2, illustrating a second common type ofunequal or nonuniform magnetic particle distribution under similarconditions, in accordance with the prior art;

FIG. 4 is a view similar to FIG. 2, illustrating a third common type ofunequal or nonuniform magnetic particle distribution under similarconditions, in accordance with the prior art;

FIG. 5 is a view illustrating an opaque condition of the liquid mediumresulting from an even or uniform distribution of such particles; and

FIG. 6 is a view similar to FIG. 2 but illustrating diagrammatically theeven or uniform particle distribution in the opaque condition of FIG. 5,in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Basic elements of themagnetic mixer shown in FIG. 1 include a magnet 10, here illustrated asof the permanent type and as being of bar shape. Magnet 10 mayconveniently take the form of an Alnico magnet, having a diameter ofapproximately one-half inch and a length of 2% inches. As will beapparent, the poles of the magnet are at the respective ends thereof,and the magnet generates a multiplicity of lines of force or flux, twotypical lines of force being indicated at 12 and 14. The magnet isarranged horizontally and is supported for rotation on a vertical shaft16 angularly fixed thereto, and having its axis as close as possible tothe midpoint between the magnet ends or poles. The axis of the magnet ishorizontal as previously indicated, and the magnet is driven through theshaft 16, as by any conventional device such as electrical motor 18, togenerate a rotary magnetic field.

Spaced a short distance above the magnet 10 is a stationary support fora volume of liquid, the contents of which are to be mixed, and whichsupport is indicated generally at 20. The support 20 may have anydesired configuration to support the volume of liquid to be mixed, butis here illustrated as having a flat upper surface portion for thesupport of the volume of liquid here illustrated as a droplet indicatedat 22. The droplet 22, which combines plural liquids to be mixed,contains in suspension a multiplicity of magnetic particles. Theseparticles are magnetized and may be formed of iron oxide and may have agenerally acicular configuration with a major dimension in the range'of0.40 to 0.60 microns, for example. Alternatively, such particles may beconstituted by cobalt or nickel materials.

The droplet 22 is illustrated as having its center on the axis ofrotation of the magnet 10. It is to be noted that the flux or force line14 is substantially flat in the area thereof passing through the droplet22. This flatness in this area is desirable to eliminate any significantvertical component of this magnetic field in the droplet 22.

The illustrated apparatus thus far described is in accordance with theteaching of the aforementioned Adler U.S. Pat. No. 3,650,698, thedisclosure of which is incorporated by reference, and like thedisclosure of the last-mentioned patent, is illustrated as embodied inapparatus for determining the coagulation rate of human blood specimens.More specifically, such coagulation-rate testing is illustrated here andin that patent as involving the determination of Prothrombin Time (PT).

For this use, illustrated by way of example, the support, generallyindicated at 20, for the liquid droplet 22 to be analyzed includes arigid horizontally elongated strip 24 of a nonmagnetic material and massappropriate for providing a heat sink. An electrical heater coil 25 isconveniently supported and located adjacent to the bottom of the heatsink 24 to generate heat for application to the heat sink which ispreferably maintained at the substantially constant temperature ofapproximately 37.5 C. The heat sink 24 has a flat upper surface, and inthe central region thereof has a mirror element 26 recessed therein soas to be substantially flush with the last-mentioned surface. The mirrorelement 26 extends beyond the periphery of the droplet 22.

The support 20 further includes a strip-like web or tape of transparentflexible material, which may be conveniently formed of Mylar, whichdirectly supports the droplet 22 above the heat sink 24 and mirrorelement 26 on the upper surface of the tape, the lower surface of thetape being supported directly by the heat sink 24 and, in the area ofthe droplet 22, the mirror element 26 in the manner shown. The tape 28may have at spaced intervals along its length thermo-formed wells 30,each of the wells 30 being provided to contain plural liquids to bemixed together. However, the wells 30 may be omitted if desired toimprove thermal transfer between the heat sink 24 and the volumes ofliquid supported on the tape. In other words, the upper surface of thetape 28 may be flat throughout the entirety of such surface and stillafford proper support for the liquid volumes.

In each of the wells 30 there is deposited, in a manner similar to thatdescribed and illustrated in the aforementioned Adler U.S. Pat. No.3,650,698, a spot (not shown) of an iron oxide material or the likewhich is dried therein during the manufacture of the tape. However it isto be noted that the amount of such iron oxide particles in each well 30is significantly lower than that described in the aforesaid Adler U.S.Pat. No. 3,650,698. In that patent, it is indicated that in themanufacture of the tape, the reaction intensifying agent deposited onthe tape, may consist of a suspension of approximately 50 grams ofmagnetic iron oxide particles in ml. of carrier solution. When themagnetic mixer of the present invention is employed in such bloodtesting the reaction intensifying agent deposited on the tape in asimilar manner, may consist of a suspension of less than 40 grams ofmagnetic iron oxid particles in 100 ml. of carrier solution.

According to the disclosure of the aforementioned Adler U.S. Pat. No.3,650,698, with reference to the details of the blood-testing apparatus,which details do not require detailed description or illustration here,the system employed with the magnetic mixer of the present invention foranalysis of Prothrombin Time or PT may include an intermittentlyoperated drive mechanism to drive the tape 28 in the direction of thearrows of FIG. 1, a sample discharge probe 32 at a first dispensingstation (FIG. 1) over the illustrated tape run, an a reagent dischargeprobe 34 at a second station over the tape run. The aforementionedsystem further includes a light source in the form of a lamp 36 over thetape run, light rays from which strike the droplet 22 at a third stationover the mirror element 26, and a photosensitive device in the form of aphotocell 38 over the tape run, which photocell coacts with the droplet22 and the mirror element 26 below the droplet.

The operation of the apparatus thus far described, detailed in theaforementioned Adler U.S. Pat. No. 3,650,698 with the there recitedamounts of the reaction intensifying agent predeposited on the tape,need not be discussed in detail here. It is sufficient here that thetape 28 is advanced to present one of the tape wells 30 at the firststation in registry with and below the dispensing probe 32 at whichstation a precisely measured quantity of a blood plasma sample is addedto the aforementioned reaction intensifying or enhancing agent, torapidly re-suspend the latter, with the effect of the aforementionedrotating magnetic field being to promote initial blood plasmasample-reaction intensifying agent mixing and render the resultantmixture subheat sink 24.

After the expiration of a period of time predetermined to provide forsample incubation, the tape 28 is advanced to present the last-mentionedtape well 30 at the second dispensing station at which the well is inregistry with and below the dispensing probe 34, at which station aprecisely measured quantity of thromboplastin reagent is added to thesubstantially turbid or opaque mixture to commence the coagulationreaction with the effect of the rotating magnetic field on the magneticiron oxide agent further promoting mixing of the sample and thethromboplastin reagent. The thromboplastin reagent dispensedby the probe34 may be diluted at the second station with calcium chloride or calciumchloride may be added to the mixture at a subsequent station not shown.After the addition of the last reagent, the tape 28 is advanced topresent the lastmentioned tape well at the third station at which thetape travel is stopped with the last-mentioned well and the liquidmixture therein in the position of the droplet 22 of FIG. 1, and thephotocell 38 and the timer are activated.

In the last-mentioned position of the sample mixture, the center of thedroplet 22 closely coincides with the center of the rotating magneticfield. As the end point of the PT or coagulation reaction is reached,the substantially turbid or opaque treated sample undergoes a sharp anddramatic change in an optical property due to the collection of therotating magnetic iron oxide particles by one or more of the fibrinstrands generally centrally of the mixture, to thus render the mixturesubstantially transparent for immediate detection by the photocell 38which detects the light directed through the droplet 22 to the cell 38by the mirror element 26. The signal generated by the photocell 38 inthis manner effects a printed readout of the Prothrombin Time or PT ofthe blood plasma sample of interest. Operation of the apparatus iscontinuous in the manner described until each of the blood plasmasamples of a series of such samples from different patients has beendetermined.

During each such test and before the production of fibrin in the treatedsample, the magnetic field of the rotating magnet spins the magneticparticles in the treated sample, substantially about the axis ofrotation of the magnet 10, creating a centrifugal force on such magneticparticles such as to tend to move these particles outwardly from thecenter of the mixture. When the population of the magnetic particles ofeach treated sample is reduced, as hereinbefore suggested for thereasons given, such a reduction under the aforementioped conditionsoften results in a lack of uniformity of particle concentrationsthroughout the treated sample with concomitant transparency in one ormore portions of the treated sample. Such transparency of the treatedsample, or lack of uniform opaqueness of such sample, results insufficient light being received by the photocell 38 to generate a signalwhich results in a false or early cutoff of the aforementioned timer,resulting in a faulty. test.

Such a redistribution of the magnetic particles in the treated, samplemay be of the type shown diagrammaticall in FIG. 2 wherein there appearsto be a rivulet 40 oft nsparent fluid flowing through the center of thetreated, sample. It is theorized that such a redistribution may takeplace through the influence of the rotary magnetic field by a pumpingaction on the fluid of certain magnetic particles which haveagglomerated in the mixture. Another type of unequal redistribution ofmagnetic particles is indicated diagrammatically in FIG. 3, whereincertain of such magnetic particles, acting under the influence of therotating magnetic field, have moved outwardly from the center of thetreated sample, leaving the entire central portion of the lattertransparent, as at 42. In FIG. 4, there is diagrammatically illustrateda further common type of nonuniformity of distribution of the magneticparticles which has been observed in a treated sample under theinfluence of a rotating magnetic field. Here the magnetic particles haveleft a transparent ring 44 around the center of the treated sample,giving the sample a bull's-eye effect.

The magnetic particle redistribution of FIGS. 2, 3, and 4, all result ina lack of uniform opaqueness of the treated sample, prior to the endreaction to be detected by the photosensitive device. The desiredopaqueness of the treated sample resulting from an even or substantiallyuniform distribution of magnetic particles is indicated at 46 in FIG. 5,while the substantially even or equal distribution of such particles,which achieves the opaque effect of FIG. 5, is indicateddiagrammatically in FIG. 6. i

To achieve the desired opaqueness of the treated sample prior to the endreaction by the substantially uniform distribution of FIG. 6, there isprovided another magnet 48 illustrated in FIG. 1 as being of thepermanent type and of bar shape. The magnet 48 is vertically arranged,its poles being illustrated in vertically spaced relation to oneanother, which magnet 48 has a vertical magnetic field indicated bytypical flux lines or force lines 50, 52.

The magnet 48 may be conveniently formed as an Alnico 5 magnet having1/4 inch diameter and a length of 1-1/4 inches. The axis of the magnet48 should closely coincide with the rotational axis of the magnet 10,and the spacing of the lower pole of the magnet 48 from the uppersurface of the tape 28 may approximate the spacing of the upper surfaceof the magnet 10 from'the upper surface of the tape. In actual practice,this spacing may approximate 0.4 inch. As clearly shown, the magneticfieldof the magnet 48 extends into the treated sample droplet 22 on thesupport 20, and extends into the magnetic field of the magnet 10, so asto overlap the last-mentioned magnetic field.

The ends of the magnet 48 protrude from the respective ends of asupporting sleeve 54 of non-magnetic material, which is externallythreaded as shown in FIG. 1, the magnet 48 being conventionally fixed tothe sleeve 54. The sleeve 54 has a knurled radial flange which -may beformed as an integral part thereof, the flange being indicated at 56.The threaded sleeve 54 carrying the magnet 48 is threaded into an eye 58extending horizontally and in fixed relation from a support bracket 60,which may have attachment ears for receipt of suitable fasteners 62.

It will be understood from the foregoing that the magnet 48 may bevertically adjusted to increase or decrease the magnetic gradient of itsmagnetic field in the area of the supported volume of liquid on thesupport 20. In other words, the last-mentioned magnet may be adjustedtoward and away the last-mentioned support and the volume of liquidsupported thereby. It will be obvious that this adjustment isaccomplished by manually turning the knurled portion 56 of the sleeve inone direction or the other. The magnet 48 is preferably verticallyadjusted to a position in which the magnetic gradient of its field inthe area of the liquids to be mixed is of sufficient strength togenerate magnetic centering forces large enough to effectively opposethe tendency of the magnetic particles in the solution under theinfluence of the rotating field of magnet 10 to move away from thecenter of the volume of liquid. In practice if it is found for onereason'or another that it is desirable to have the central portion ofthe volume of liquid more opaque than the peripheral region thereof, themagnet 48 may be adjusted so that the vertical component of its field inthe area of the volume of liquid produces forces which counter balancethe tendency of the rotating magnetic field to redistribute the magneticparticles outwardly, so that the magnetic particles have some tendencyto collect toward the center of the volume of liquid.

It will be evident from the foregoing that the construction andarrangement of the magnets 10 and 48 is such that their combinedmagnetic fields may effect a substantially even or uniform distributionof magnetic particles throughout combined liquids, which liquids arebeing mixed by the effect of one of the magnets through its rotatingfield, which combined effect of the magnetic fields enhances mixing andeffectively opposes centrifugal force acting on the magnetic particlestending to redistribute such opaque particles from the center of thevolume of liquid toward its periphery. It will be further understoodthat in the combination of the two magnetic fields illustrated anddescribed, a lesser concentration of magnetic particles for mixing maybe utilized than heretofore to provide the opaque effect of FIG. 5 ofthe volume of liquid which is achieved by the substantially evendistribution of the magnetic particles in solution.

Although disclosed in the form of a preferred embodiment which isdirected toward the automatic successive determination of the respectiveProthrombin Times or PT of a plurality of blood plasma samples, it isbelieved that it has been made clear that the apparatus of the inventionhas significant utilization for different purposes. More specifically,the apparatus may be utilized, for example, for the determination ofPartial Thromboplastin Time of blood plasma samples to enable the usethereof for the more specific isolation of the fact or factors causingdeficiencies in the clotting time of a patient's blood. As anotherexample, the apparatus, with suitable modification, may be utilized inthe determination of fibrinolysis of coagulated blood plasma samples. Inaddition, and again with suitable modification, it is believed that itwill be apparent to those versed in the art that the apparatus of myinvention may be utilized in the determination of the end point of apolymerization-type reaction, the end point of which is evidenced by anabrupt change in viscosity, in a wide variety of liquids other than anddifferent from blood plasma samples. Thus, for example, the apparatus ofthe invention may be utilized to determine the end point of aflocculation reaction as occurs in pregnancy testing, or in the endpoint of an agglutination reaction such as occurs in testing forrheumatoid arthritis.

It is to be noted that the support for the tape 28 which in turnsupports discrete, volumes of liquid thereon may take other forms. Forexample, the tape support may take the form of that shown and describedin the Bidanset US. Pat. No. application, Ser. No. 160,858, filed July8, 1971, assigned to the assignee of the instant application, whichdiscloses a tape cassette having a platen on which there is a tape run.It is to be understood that the volume of liquid to be mixed inaccordance with the present invention, and herein illus trated as adroplet of liquid, need not be supported on a tape or the like, such asthe tape 28, but may be supported directly on the upper surface of themirror element 26.

While there has been shown and described the preferred embodiments of myinvention, it will be understood by those versed in the art that theinvention may be embodied other than as herein specifically illustratedor described, and that the invention is susceptible of taking otherforms, and that changes may be made in details without departing fromthe principles of the invention.

What is claimed is:

1. Apparatus such as described, comprising: a support for a volume ofliquid containing a suspension a multiplicity of magnetic particles,means effecting a rotary movement of said particles relatively to saidliquid about an axis, which movement tends to be characterized byredistribution and collection of said particles in one or more localizedareas in said liquid, and which rotary movement generates a centrifugalforce affecting said particles, and means externally of said liquidgenerating a magnetic field gradient coacting with said particles toovercome such redistributing effect thereon and balance said force, forsubstantial uniformity of distribution of said particles in said liquid.

2. Apparatus as defined in claim 1, wherein: said means effecting arotary movement of said particles relatively to said liquid comprises arotating magnetic field gradient generated by a magnet having a rotaryaxis on which it is driven extending centrally between poles of saidmagnet.

3. Mixing apparatus comprising: a first magnet having its axis betweenpoles of the magnet disposed in a first plane, means for rotating saidmagnet on a second axis normal to said plane to generate a rotatingfirst magnetic field, the second axis being located centrally betweensaid magnet poles, support means for fixedly supporting substantially onsaid second axis a volume of plural combined liquids to be intermixed bya suspension of magnetic particles in said liquid volume, and a secondmagnet having poles, generating a second magnetic field and having itsaxis substantially on said second axis, one of said poles of said secondmagnet being spaced nearer said liquid volume than the other of saidlast-mentioned poles, said support means supporting said liquid volumein both said first and second magnetic fields so that dislocatingmovement of said magnetic particles by centrifugal force as the lattermove through said liquid volume during mixing of the constituents latteris effectively opposed by said second magnetic field.

4. Mixing apparatus as defined in claim 3, wherein: said second magnetis a permanent magnet.

5. Mixing apparatus as defined in claim 3, wherein: said first andsecond magnets are permanent magnets.

6. Mixing apparatus as defined in claim 3 further comprising means toadjustably shift said second magnet and said support means relativelyapart or toward one another.

7. Mixing apparatus as defined in claim 3, further comprising aphotosensitive device cooperating with said liquid volume to indicate anoptical change in said liquid volume during mixing of its constituents.

8. Mixing apparatus as defined in claim 3, wherein: said first magneticfield is horizontally arranged and said second magnetic field isvertically arranged.

9. Mixing apparatus as defined in claim 3, wherein: said support meanscomprises a tape for intermittent travel in a direction transverse tosaid second magnetic field which tape supports said liquid volume in theshape of a droplet on a surface of the tape.

10. Mixing apparatus as defined in claim 9, wherein: said first magneticfield is horizontally disposed and said second magnetic field isvertically disposed, said second magnet being disposed in spacedrelation above said first magnet l l. Mixing apparatus as defined inclaim 10, wherein: said support means comprises a tape for intermittenttravel in a horizontal direction, which tape supports said liquid volumein the shape of a droplet on an upper surface of the tape.

12. Apparatus such as described, comprising: a supthereon, to achievesubstantial uniformity of distribution of said particles throughout saidliquid volume.

13. A method of mixing a volume of plural combined I liquids to beintermixed by activation of magnetic particles in suspension in theliquid volume comprising: the step of generating a rotating firstmagnetic field, generating a second magnetic field substantially atright angles and centrally of said first magnetic field so as to have aportion thereof extending into and overlapping said first magneticfield, and supporting said liquid volume in said overlapping portion ofsaid second magnetic field, so that said magnetic particles areattracted by both magnetic fields.

1. Apparatus such as described, comprising: a support for a volume ofliquid containing a suspension a multiplicity of magnetic particles,means effecting a rotary movement of said particles relatively to saidliquid about an axis, which movement tends to be characterized byredistribution and collection of said particles in one or more localizedareas in said liquid, and which rotary movement generates a centrifugalforce affecting said particles, and means externally of said liquidgenerating a magnetic field gradient coacting with said particles toovercome such redistributing effect thereon and balance said force, forsubstantial uniformity of distribution of said particles in said liquid.2. Apparatus as defined in claim 1, wherein: said means effecting arotary movement of said particles relatively to said liquid comprises arotating magnetic field gradient generated by a magnet having a rotaryaxis on which it is driven extending centrally between poles of saidmagnet.
 3. Mixing apparatus comprising: a first magnet having its axisbetween poles of the magnet disposed in a first plane, means forrotating said magnet on a second axis normal to said plane to generate arotating first magnetic field, the second axis being located centrallybetween said magnet poles, support means for fixedly supportingsubstantially on said second axis a volume of plural combined liquids tobe intermixed by a suspension of magnetic particles in said liquidvolume, and a second magnet having poles, generating a second magneticfield and having its axis substantially on said second axis, one of saidpoles of said second magnet being spaced nearer said liquid volume thanthe other of said last-mentioned poles, said support means supportingsaid liquid volume in both said first and second magnetic fields so thatdislocating movement of said magnetic particles by centrifugal force asthe latter move through said liquid volume during mixing of theconstituents latter is effectively opposed by said second magneticfield.
 4. Mixing apparatus as defined in claim 3, wherein: said secondmagnet is a permanent magnet.
 5. Mixing apparatus as defined in claim 3,wherein: said first and second magnets are permanent magnets.
 6. Mixingapparatus as defined in claim 3 further comprising means to adjustablyshift said second magnet and said support means relatively apart ortoward one another.
 7. Mixing apparatus as defined in claim 3, furthercomprising a photosensitive device cooperating with said liquid volumeto indicate an optical change in said liquid volume during mixing of itsconstituents.
 8. Mixing apparatus as defined in claim 3, wherein: saidfirst magnetic field is horizontally arranged and said second magneticfield is vertically arranged.
 9. Mixing apparatus as defined in claim 3,wherein: said support means comprises a tape for intermittent travel ina direction transverse to said second magnetic field which tape supportssaid liquid volume in the shape of a droplet on a surface of the tape.10. Mixing apparatus as defined in claim 9, wherein: said first magneticfield is horizontally disposed and said second magnetic field isvertically disposed, said second magnet being disposed in spacedrelation above said first magnet.
 11. Mixing apparatus as defined inclaim 10, wherein: said support means comprises a tape for intermittenttravel in a horizontal direction, which tape supports said liquid volumein the shape of a droplet on an upper surface of the tape.
 12. Apparatussuch as described, comprising: a support for a volume of liquidcontaining in suspension a multiplicity of magnetic particles, meanseffecting movement of said particles relatively to said liquid volume,which movement tends to be characterized by redistribution andcollection of said particles in one or more localized areas in saidliquid volume resulting in a non-uniform particle distributionthroughout the liquid volume, and means externally of said liquidgenerating a magnetic field gradient coacting with said particles toovercome such non-uniform distributing effect thereon, to achievesubstantial uniformity of distribution of said particles throughout saidliquid volume.
 13. A method of mixing a volume of plural combinedliquids to be intermixed by activation of magnetic particles insuspension in the liquid volume comprising: the step of generating arotating first magnetic field, generating a second magnetic fieldsubstantially at right angles and centrally of said first magnetic fieldso as to have a portion thereof extending into and overlapping saidfirst magnetic field, and supporting said liquid volume in saidoverlapping portion of said second magnetic field, so that said magneticparticles are attracted by both magnetic fields.